1. Field of the Invention
The invention relates to crosslinkable compositions having controllable modulus, and based on organosilicon compounds, to a process for their preparation, and also to their use.
2. Background Art
There are a wide variety of known crosslinkable compositions based on organosilicon compounds. Single-component sealing compounds which can be stored if water is excluded, but on ingress of water vulcanize at room temperature to give elastomers, are likewise known. Large amounts of these products are used, for example, in the construction industry. Such sealing compositions are based on linear polymers terminated by silyl groups bearing reactive substituents such as OH groups, or hydrolyzable groups, e.g. acetoxy groups. These mixtures also usually comprise fillers, non-reactive plasticizers, and, if appropriate, crosslinking agents, catalysts, and additives. For many applications it is desirable that the modulus of a hardened test specimen of the sealing compound, i.e. the tensile stress value at 100% tensile strain, is low. This minimizes the tensile stress on the side of the joint when a joint is subjected to tensile strain, thus avoiding any breaking away of the sealing compound from the substrate. The large chain lengths specifically required for low-modulus sealing compounds lead to very high viscosities of the polymers, the result being difficulty in handling of the sealing compounds in the non-crosslinked state. For example, the materials are difficult to extrude from packs, or exhibit an undesirable extent of stringiness during use.
It has also been known for a long time that the modulus can be varied by varying the amount of non-reactive plasticizers and of active fillers. However, there are limits to which variation of these components can be made; not every desired variation is possible. If, for example, the amount of plasticizer is excessive, or the amount of active filler is inadequate, the viscosity of the composition becomes too low, impairing the application performance of the paste-like sealing compound. Use of high-viscosity polymers which to some extent contain an unreactive group at a chain end is also known. By way of example, U.S. Pat. No. 6,235,832 describes polymers which are obtained via reaction of vinyl-terminated polydimethylsiloxanes with Si—H-containing crosslinking agents. However, there is then the risk that unreactive, high-viscosity constituents will exude from the hardened sealing compound, which can lead to soiling of the subtrates. U.S. Pat. No. 3,542,901 describes the use of a preferably high-viscosity linear polysiloxane in an alcohol-eliminating RTV1 system which has random distribution of reactive and unreactive end groups, the permissible total amount of unreactive chain ends present being only at most 40%. However, this leads to products with undesirably high vulcanisate haidnesses.